gitweb: Add optional output caching

gitweb: Add optional output caching

This commit actually adds output caching to gitweb, as we have now
minimal features required for it in GitwebCache::SimpleFileCache
(a 'dumb' but fast file-based cache engine).  To enable cache you need
(at least) set $caching_enabled to true in gitweb config, and copy
required modules alongside generated gitweb.cgi - this is described
in more detail in the new "Gitweb caching" section in gitweb/README.
"make install-gitweb" would install all modules alongside gitweb
itself.

Caching in theory can be done using any Perl module that implements
Cache::Cache compatibile get/set (method) interface.  The default is
to use GitwebCache::SimpleFileCache.  Capturing and caching output
is done via cache_output subroutine from GitwebCache::CacheOutput.

The cache_output subroutine in GitwebCache::CacheOutput currently uses
GitwebCache::Capture::Simple compatibile capturing engine passed as
one of parameters to cache_output subroutine.  The default is to use
GitwebCache::Capture::Simple package.

Capturing and caching is designed in such way that there is no
behaviour change if $caching_enabled is false.  If caching is not
enabled, then capturing is also turned off.

Enabling caching causes the following additional changes to gitweb
output:
* Disables content-type negotiation (choosing between 'text/html'
  mimetype and 'application/xhtml+xml') when caching, as there is no
  content-type negotiation done when retrieving page from cache.
  Use lowest common denominator of 'text/html' mimetype which can
  be used by all browsers.  This may change in the future.
* Disable optional timing info (how much time it took to generate the
  original page, and how many git commands it took), and in its place show
  unconditionally when page was originally generated (in GMT / UTC
  timezone).
* Disable 'blame_incremental' view, as it doesn't make sense without
  printing data as soon as it is generated (which would require tee-ing
  when capturing output for caching)... and it doesn't work currently
  anyway.  Alternate solution would be to run 'blame_incremental' view
  with caching disabled.

Add basic tests of caching support to t9500-gitweb-standalone-no-errors
test: set $caching_enabled to true and check for errors for first time
run (generating cache) and second time run (retrieving from cache) for a
single view - summary view for a project.

Check in the t9501-gitweb-standalone-http-status test that gitweb at
least correctly handles "404 Not Found" error pages also in the case
when gitweb caching is enabled.

Check in the t9502-gitweb-standalone-parse-output test that gitweb
produces the same output with and without caching, for first and
second run, with binary or text output.

All those tests make use of new gitweb_enable_caching subroutine added
to gitweb-lib.sh

Inspired-by-code-by: John 'Warthog9' Hawley <warthog9@kernel.org>
Signed-off-by: Jakub Narebski <jnareb@gmail.com>